In a quadrature signal system, baseband signal may comprise two-real signals: in-phase (I) and quadrature-phase (Q) signals. These I and Q baseband signals are multiplied with cosine and sine waves of a RF transmitter and combined to generate a RF passband signal (IQ RF signals). Ideally, the cosine and sine waves of the RF transmitter have the same amplitude and differ in phase by 90 degrees, thereby making the IQ RF signals a pair of quadrature signals. Zero-IF receiver employs homodyne or direct down conversion method to receive this pair of quadrature signals. During the direct down conversion, the passband signal is mixed with the in-phase and quadrature-phase components of a local oscillator signal to generate IQ baseband signals for further baseband processing.
While receiving the IQ RF signals, it is important to maintain the amplitude and phase relationship between the I and Q signals to ensure an accurate signal reception. It is also important to maintain the same gain and the 90 degree phase relationship between the in-phase and quadrature-phase components of the local oscillator to prevent a gain or phase skew between the I and Q signals. In reality, however, errors such as an IQ gain/phase imbalance existing in a zero-IF receiver impairs the amplitude and phase relationship between the IQ RF signals. Such IQ gain/phase imbalances are due to mismatches in the gain and phase between the components of the local oscillator, and the mismatches in the analog filters and analog-digital converters (ADC) between the I and Q paths.
A correction is attempted to compensate for impairments caused by IQ mismatch by estimating the IQ mismatch. The zero-IF receiver may, in turn, compensate for skews on the pair of quadrature signals by correcting the quadrature signals based on the IQ mismatch estimates.